In the printing industry, it is known to print on a continuous web while it is transported through a one-directional print engine. Such a one-directional print engine, for example, provides a print medium, such as ink or toner, on one side of the continuous web, typically the top side, while it is transported along the print engine. Such a print engine can comprise a single print head for a single color print or a plurality of print heads for multicolor prints. For multicolor prints typically at least four print heads are provided using the colors cyan, magenta, yellow and carbon (black).
If double-sided printing on the continuous web is desired by such a one-directional print engine, it is necessary to turn or flip the web by 180° to permit a second pass of the web through the one-directional print engine, in such a state that the previously non-printed side of the web faces the respective print head(s) of the print engine.
To perform this turning function, it has been known to utilize a so-called “turn-bar arrangement”, which is typically made up of several stationary turn bars, which are angled with respect to a paper transport path to provide a turning function. Inasmuch as the turn bars are typically stationary, contact between the web and the turn bar should preferably be reduced to a minimum, in order to avoid vibration of the turn bar or the web. Therefore, such turn bars are typically provided with through holes, to permit air to be blown through the turn bar, in order to generate an air cushion between the turn bar and a web while it is traveling around the turn bar.
Despite the fact that such an arrangement permits both a turning and a lateral shifting of the web, such system is rather expensive and causes a lot of noise due to the blowing of air through the turn bar. Furthermore, the air cushion between the turn bar and the web is not always stable, thus, causing problems in appropriately guiding the web through the print engine.
An alternative turning device for a continuous web is for example known from IBM Technical Disclosure Bulletin, Vol. 22, No. 6, Nov. 19, 1979 page 2465. In this turning device a continuous web is guided after having been printed on a first side around a first roller, a second roller oriented at 90° with respect to the first roller and a third roller arranged at an angle of 90° to the second roller, i.e. parallel to the first roller. The web is guided around these rollers in such a manner that between the first and second roller and the second and third roller the web performs a 90° twist. The second roller is arranged approximately at a midpoint between the first and second rollers, such that turning and laterally shifting of the web is achieved. This arrangement, however, causes problems with properly guiding the web, inasmuch as the second roller will cause a tendency of the web to move towards the diverting line between the second and third rollers. There is, therefore, a continuing need for ways of turning a web.